What's your best purchase ever?

Parallax VX3 or Blackhand. I've run up 20+ streaks with primarily stalker and Blackhand several times - it's basically a sniper rifle in a pistol; line up the headshot, and if that hits, any further hit is generally a kill, and it's great at range. The Parallax *has* to be the best sniper rifle in the game - hits hard, one shots are easy, and you can chamber and fire another round while in scope - it's all win, IMHO.